Membranes from in vivo labeled cells of Rhodobacter capsulatus U43[pTX35] grown photosynthetically carried 60% of the [P-32]-Pi in the ''heavy'' fraction (HM) after sucrose gradient sedimentation, Metal-chelating chromatography of either ''heavy'' or ''Light'' (LM) membrane fractions rendered similar Bchl-protein complex profiles after octyl-glucoside treatment, including most of the radioactivity in the same corresponding elution fraction (F II). Similar labeling distribution of pigment-protein complexes was obtained for membranes of dark-grown cells induced by lowering oxygen tension. Fractions derived from HM showed highly labeled LHI alpha, whereas the same complex from LM was essentially [P-32]-Pi-free, as revealed by SDS PAGE followed by autoradiography. Phospholipid analysis showed a similar pattern for membranes isolated from cells photosynthetically or semiaerobically grown, being the most abundant: phosphatidyl glycerol, phosphatidylethanolamine, cardiolipin, and phosphatidyl-choline. Part of the phospholipids from HM comigrated with LHI alpha during SDS-PAGE and dissociated from the complexes only after solvent extraction and hydrophobic chromatography, However, a small amount remained always attached to LHI alpha, indicating an unusual strong interaction. These results suggest the existence of two operationally defined membrane regions carrying LHI alpha complexes differing in phosphorylation status and protein-phospholipid interaction.